Conventional overvoltage protection devices typically use a gas discharge surge arrestor, or “gas tube,” as a primary means for diverting voltage surges from a signal line to ground. Examples of such devices are shown in U.S. Pat. Nos. 5,388,023, 5,500,782 and 5,880,919. Gas tubes dissipate energy by causing electrical arcing to ground. A gas of known dielectric strength is ionized when subjected to an electrical surge. One drawback of gas tubes, however, is that they typically exhibit a relatively slow response time, and thus, may not be able to safely suppress fast rise time voltage surges. Accordingly, metal oxide varistors (MOVs) have been employed as secondary protectors in back-up and interacting overvoltage protection devices. For example, in a conventional hybrid station protector, an MOV is electrically connected in parallel with the gas tube between each signal line and electrical ground. Although the gas tube can repeatedly dissipate voltage surges without damage, the response time of the MOV is substantially faster than the response time of the gas tube. Therefore, the MOV can be relied upon to shunt fast rise time voltage surges to ground, while the gas tube is relied upon to shunt sustained voltage surges, which might otherwise damage the MOV.
Overvoltage protection devices utilizing MOVs as secondary protectors have been successfully employed to protect conventional twisted-pair (i.e., “tip” and “ring”) telephone lines. Broadband communications, such as digital subscriber line (DSL) transmissions, which are generically referred to herein as “xDSL”, operate at transmission frequencies that are substantially higher than the frequencies traditionally employed over twisted-pair telephone lines. Presently, frequencies of at least 1 megahertz, and generally about 30 megahertz, are utilized for xDSL communications transmitted over twisted-pair telephone lines. Existing twisted-pair telephone lines, also referred to as outside plant wire, are typically CAT-3 grade or less and were not intended for high frequency performance when originally manufactured or installed. In many instances, conventional overvoltage protection devices are inadequate for higher frequency digital transmissions, for example VDSL. Even if only a small number of overvoltage protection devices perform inadequately, the cost of identifying and replacing the overvoltage protection devices that may be adequate for lower frequency xDSL communications, but inadequate for higher frequency xDSL communications, is significant.
The inadequate performance of some conventional overvoltage protection devices, such as station protectors utilized at customer premises for higher frequency xDSL communications, has been traced to the relatively high capacitance and the variability of the capacitance of the MOVs that are employed in the station protector. At higher frequencies, the capacitance and the variability of the capacitance results in unacceptable insertion loss, return loss, and longitudinal imbalance. It is well known that the capacitance can be reduced by utilizing MOVs of the same thickness, but having a smaller diameter. Many conventional station protectors employ 5 mm diameter MOVs with 3.8 mm electrodes on either side of the varistor material instead of smaller diameter MOVs because the larger diameter MOVs absorb additional energy without permanent damage. MOVs of this size with symmetrical electrodes have a capacitance of about 60 picofarads with a tolerance of about 20% (i.e., 60 picofarads ±12 picofarads). This relatively large tolerance is believed to be due to variability in the varistor material and thickness, and/or to the relative placement and size of the electrodes on opposite sides of the varistor material. Electrodes, which are intended to be aligned on opposite sides of the varistor material, can in practice be laterally displaced relative to each other. The concentricity of the two electrodes can also vary. Lateral displacement and varying concentricity of the electrodes on opposite sides of the varistor material means that the overlapped surface area of the electrodes can vary significantly between MOVs that are intended to be identical, thereby generating dissimilar electric fields that result in relatively high capacitive tolerance. The difference in the capacitances of the MOVs utilized between the tip conductor and ground and between the ring conductor and ground results in significant capacitance mismatch, referred to herein as “capacitive imbalance.” In turn, excessive capacitive imbalance can cause unacceptable signal loss (e.g., insertion loss and return loss) and longitudinal imbalance at the higher frequencies utilized for xDSL communications transmitted over twisted-pair telephone lines.
As previously mentioned, it would be possible to reduce the capacitance between a signal line and ground in a station protector by utilizing an MOV of the same thickness having a smaller diameter. Because the electrodes of the smaller diameter MOV inherently have a smaller overlapped surface area, the smaller diameter MOV also has less capacitance. However, a smaller diameter MOV is not able to withstand the same sustained current as a larger diameter MOV. Furthermore, substitution of the smaller diameter MOV would result in significant engineering, re-tooling and testing expense. Even if the desired reduction in capacitance could be achieved by substituting a smaller diameter MOV for the 5 mm MOV presently in use, there could still be an excessive capacitive imbalance between the tip conductor and ground and the ring conductor and ground. Accordingly, it would be preferable if a reduction in the capacitive imbalance could be achieved without the need for extensive modifications to the present design of existing station protectors.
The number of station protectors and other overvoltage protection devices manufactured that are incapable of adequate performance at higher frequencies is significant, at least in the aggregate. In particular, when MOVs having relatively high capacitance and large capacitive tolerance are employed in twisted-pair telephone lines, an excessive imbalance in the capacitance between the tip conductor to ground signal line and the ring conductor to ground signal line will be present in a significant number of station protectors. In fact, a final inspection rejection rate as much as 10% is not uncommon. The capacitive imbalance for such station protectors has been found to be up to about 5 picofarads. For xDSL communications, a capacitive imbalance of less than about 1.3 picofarads is desired. Accordingly, what is needed is an overvoltage protection device in which the capacitive imbalance due to the capacitive tolerance of the MOVs utilized in the device is reduced, but in which the same current can be sustained without permanent damage to the MOVs. Such overvoltage protection devices, for example station protectors, could then be utilized without introducing an excessive capacitive imbalance in twisted-pair telephone lines that transmit higher frequency xDSL communications.